Thermal spraying, also known as flame spraying, involves the heat softening of a heat fusible material such as metal or ceramic, and propelling the softened material in particulate form against a surface which is to be coated. The heated particles strike the surface where they are quenched and bonded thereto. A conventional thermal spray gun is used for the purpose of both heating and propelling the particles. In one type of thermal spray gun, the heat fusible material is supplied to the gun in powder form. Such powders are typically comprised of small particles, e.g., between 100 mesh U. S. Standard screen size (149 microns) and about 2 microns.
A plasma spray gun such as disclosed in U.S. Pat. No. 4,674,683 utilizes an arc generated plasma flame to produce the heat for melting of the powder particles. The primary plasma gas is generally nitrogen or argon, and hydrogen or helium is usually added to the primary gas. The carrier gas for transporting powder is generally the same as the primary plasma gas, although other gases may be used in certain situations. A plasma spray gun basically comprises a rod-shaped cathode and a tubular nozzle-anode connected to sources of power and plasma-forming gas. The high temperature plasma stream flows axially from the nozzle. Various configurations have been disclosed for auxiliary annular gas flows around the plasma stream for such purposes as shrouding and cooling; typical arrangements are shown in U.S. Pat. Nos. 2,922,869, 4,389,559, 4,558,201 and 4,777,342.
Powder injection into a plasma gun for spraying a coating must be effected from the side of the plasma stream because of the preemptive presence of the centrally located cathode. There is a tendency for a small amount of the powder to adhere to nozzle surfaces, resulting in buildup which can interfere with the spraying and coating. For example buildup on one side can cause the spray stream to skew, or a piece of the buildup may break off and deposit as a defect in the coating.
Buildup is reduced significantly by feeding the powder into the stream externally with a lateral powder injector as shown in the above mentioned U.S. Pat. No. 4,674,683. However, even this type of feed sometimes results in detrimental buildup on the nozzle face near the injector. Moving the injector away from the nozzle helps, but at a sacrifice of heating efficiency to the powder.